LONDON, April 16 (Reuters) - The European Medicines Agency warned on Wednesday that vials of Roche's breast cancer drug Herceptin, thought to have been stolen from hospitals in Italy, have been tampered with and reintroduced into the supply chain.

"Italian law enforcement authorities are currently investigating the theft and are looking at whether other medicines may also be affected," the agency said in a brief statement.

So far there have been no reports that any harm has come to patients who may have been given the stolen drug, which was sold under false credentials.

Healthcare professionals have been alerted to the falsified vials, which are labelled as Italian Herceptin 150 mg, the agency said.

In a separate statement Roche said that counterfeit Herceptin has been discovered in Britain, Finland and Germany.

A chemical analysis of one of the bogus vials found that the product did not contain the injectable cancer medicine's active ingredient, the company said. In other cases, there was evidence of tampering or dilution although the vials did contain Herceptin, it said.

"Such tampering could compromise the sterility or efficacy of the product putting the health and wellbeing of patients at risk," Roche said.

The Basel-based firm said it was recalling all vials suspected of being falsified as a precautionary measure.

Roche's drugs have been targeted by counterfeiters in the past. In 2012 bogus copies of its cancer drug Avastin entered the supply chain in the United States and Europe.